RMS velocity
Though the PIV image sequence is collected with a time spacing of 0.25 seconds, RMS
velocity fluctuations in the near wake can still be obtained from them. The reasoning
is that the near wake is dominated by large structures that in turn are associated with
large time scales. A validation of PIV measurement of RMS velocity with hotwire data
is reported in the doctoral thesis of Sunshanta Dutta available in this website.
The root mean square velocity (for a generic component u) at a location in the
wake is calculated from the formula
![](images/image170.png)
The summation extends over samples, being the number of acquired velocity fields
in time. Further, is the unsteady part of the velocity field that is given as
![](images/image173.png)
The time averaged velocity field U is determined by averaging the instantaneous velocity
vectors using the formula
![](images/image174.png)
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